Apollo astronauts have died from cardiovascular problems at a much higher rate than other astronauts. Why?

Of all the risks facing astronauts on a trip to Mars, radiation generally tops the worry list. It’s long been known that venturing outside Earth’s protective magnetic field—to the moon or Mars—exposes astronauts to a steady bombardment from heavy cosmic rays that can damage DNA and increase long-term cancer risk. And that’s not even considering the acute risk of radiation sickness if space travelers were caught in a strong solar storm without some kind of shielding.

In the past, the effects of radiation on astronauts’ cardiovascular health hasn’t gotten as much attention as the risk from cancer. That’s why a study published today in Scientific Reports by Michael Delp of Florida State University and his colleagues is troubling: Apollo astronauts—the only humans ever to venture into deep space—have died from cardiovascular disease at a rate four to five times higher than other astronauts. This is the first time anyone has looked at mortality of the Apollo astronauts as a separate group.

The researchers looked at the cause of death for three groups of deceased astronauts: 35 who never flew in space (some of them from canceled projects of the 1960s), 35 who flew in Earth orbit, and seven of the eight deceased lunar astronauts (Edgar Mitchell’s death last February came after the data were analyzed). Mortality rates due to cancer were similar for all three groups, but mortality rates from cardiovascular disease—heart attacks, strokes and the like—was strikingly different for the lunar astronauts (43 percent) than for the unflown astronauts (9 percent) or the Earth orbit astronauts (11 percent).

Delp and his colleagues then looked for a mechanism that might explain these results. They separated mice into three groups: Some underwent simulated weightlessness, others were exposed to the kind of radiation experienced by the Apollo astronauts, and a third group got both. Six to seven months later (equivalent to 20 human years), the irradiated mice showed clear evidence of artery impairment of the kind that typically leads to cardiovascular disease in humans.

“These data suggest that human travel into deep space may be more hazardous to cardiovascular health than previously estimated,” wrote the authors.

Now the caveats: The researchers realize that their sample size for the lunar astronauts is small. And the exact type of radiation the Apollo crews were exposed to—for example the proportion of light and heavy particles—is not well understood (although the lunar astronauts did report seeing light flashes during times of darkness, presumed to be charged particles hitting their retinas, at the astonishing rate of one every few minutes).

“There clearly are other factors [beside radiation] that could contribute” to the higher rates of heart disease among the lunar astronauts, says Delp. Working with researchers at NASA’s Johnson Space Center, he’s already started a more in-depth of study of Apollo astronauts’ health that could uncover some of those factors. These studies will take time, though, and Delp says there is “perhaps a little bit of urgency” to understanding the cardiovascular risks of deep-space radiation, now that the United States, Europe and China are considering sending people to the moon and Mars, beginning in 10 years or less.

Scientists would like to know more about the biological effects of radiation, but opportunities for mouse studies like the one done by Delp and his colleagues are limited by the availability of particle accelerators at the Brookhaven National Laboratory in New York. Russian scientists are planning the next satellite in their Bion series, which would place research animals in high orbits exposed to radiation, but that month-long mission won’t fly until 2020. Delp hopes that U.S. investigators will have experiments onboard.

Meanwhile, NASA and other world space agencies will have to come up with an overall approach to risk that considers all the dangers of a Mars mission, not just increased chances of getting cancer or heart disease. Some risks may just have to be accepted. As a NASA Inspector General report concluded last year: “Although the Agency plans to continue efforts to develop countermeasures to address the radiation risk, NASA is likely to seek an exception from the current standards for those that cannot be fully mitigated.”